chart-items-max-index is the
index of the last chart item of a chart-widget that should be displayed at the
current time. The value should be a non-negative integer, or
else nil for no maximum. The default value
is nil.

If set-chart-value has been
used to add items to the chart, then any item whose index is greater
than this property value (if it is a number) will not appear in the
chart. When the property value is nil, then
no items are excluded. This property can be used to display only a
subset of collected values that are of interest at the moment.

If instead a chart-value-returner function is used for
looking up chart data, then this property should always be set to a
non-negative integer to tell the chart the maximum item index that it
should pass to the chart-value-returner function. Otherwise the
chart-widget has no way of knowing how many items your function is
prepared to return. (The property will arbitrarily default to 100 in
this case, though that would usually not match the available data.)

This property is the maximum index that will be included, rather than
the number of indeces to include. So for a chart-value-returner function to cover all
values from a 100-element vector, for example, the value of this
property should be 99 rather than 100.